Earthquakes are frequent events. Dozens occur daily in the outer layer of our planet Earth, called the lithosphere; although most are too small to be noticeable, let alone newsworthy. The lithosphere consists of the upper mantle topped by the crust; it varies in thickness from approximately 55 to 200 kilometers, at its thinest under the ocean deeps. The majority of the mantle is molten or liquid rock, while the crust is solid rock; the upper mantle is a mix of the two that varies dynamically. Because of the flowing, boiling chaos below it, the crust of the Earth is not able to sustain itself as a single connected layer. Instead, it is composed of what we call tectonic plates, essentially solid connected regions of varying sizes with cracks or fractures between them. The majority of earthquakes and most of the major ones are centered in these fractures between the tectonic plates.
Our news programs, when describing an earthquake, typically tell us two things: the epicenter and the magnitude. The epicenter is the location on the Earth’s surface directly above where the earthquake occurred, no matter how deep that focal point, called the hypocenter, was. The magnitude is given using the Richter scale, which is a logarithmic, mathematical scale where each higher digit represents a 32-fold increase in the energy released at the focal point. That is, an earthquake registering five on the Richter scale is 32 times more powerful at its hypocenter than one registering four.
The Richter scale starts at zero and theoretically has no upper limit. Those measuring over 6.0 are considered major. The largest measured in recorded history occurred on May 22nd, 1960; it was 9.5 on the Richter scale and was centered on Valdivia in Chile. It killed 1655 people locally, injured upwards of 3000 and made two million homeless. Further deaths and extensive damage occurred across the Pacific Ocean and in Asia from the tsunami waves generated.
The Richter magnitude scale was first proposed in 1935 by Charles Richter, as a mathematical tool to compare earthquakes; he was working on his Ph.D. in theoretical physics at the California Institute of Technology at the time. It became readily accepted by seismologists, the scientists specializing in the study of earthquakes, and news reporters filing stories on earthquake disasters. While it continues to this day as the descriptive device used by journalists to express the strength of an earthquake, it has predominantly been replaced by the moment magnitude scale in the research on earthquakes conducted by seismologists.
The moment magnitude scale was first offered to the scientific community in 1979 by Tom Hanks (not the actor) and Hiroo Kanamori, seismologists at Harvard University. It is considered more valid for the study and comparison of medium to large scale earthquakes because, unlike the Richter scale, it does not compress the relative, measured magnitudes of larger earthquakes. This enables a greater degree of accuracy in comparing significant individual earthquake events. The moment magnitude scale is not used to measure earthquakes registering under 3.5 on the Richter scale by agencies such as the United States Geological Survey, because there is no comparable difference at this level.
Earthquake gradings by the Richter scale are a mathematical evaluation of the wave amplitude, as measured by a seismograph, of the radiated seismic energy, a small and varying subset of the total energy of the earthquake, which is referred to as the seismic moment. The moment magnitude scale is a mathematical evaluation of the amount of displacement across the fault zone that occurs as a result of the earthquake. It might be said that the Richter scale relies on subjective data where the moment magnitude scale relies on objective data. Which is why the moment magnitude scale is preferred by scientists; the Richter scale is probably preferred by the media because it’s older and more familiar to the general public, but possibly also because it is more subjective, aligning it closer to the concept of story telling.